The present invention relates to a method for fabricating a buried heterostructure semiconductor laser used as a light source for optical fiber communications.
Laser diodes are widely used as optical sources for optical fiber communications mainly because they are capable of modulating a signal at high speed. In particular, buried heterostructure semiconductor laser diodes have superior characteristics in that they have a low oscillatory threshold value and a stable oscillation transverse mode, as well as a high quantum efficiency and high characteristic temperature. This is because, in the buried heterostructure laser diodes, a current blocking layer can be formed on both sides of an active layer formed between two clad layers having a large energy gap and a small refractive index. This way, current leakage during operation is substantially reduced, if not prevented.
A conventional method for the fabrication of semiconductor laser diodes having a semi-insulating buried ridge is exemplified in FIGS. 1-7 and described bellow.
Referring to FIG. 1, the process steps for fabricating a laser diode with a buried ridge begin with the formation of a multi layered structure 100 on an n-InP substrate 10. The multi layered structure 100 is formed of a first clad layer 12 of n-InP, an active layer 14, a second clad layer 16 of p-InP, and a layer 18 of selective area growth using a quaternary material (SAC-Q), layers that are sequentially formed and successively epitaxially grown to complete a first crystal growth. The active layer 14 could be, for example, a multiple quantum well (MQW) structure formed of undoped InGaAs/InGaAsP pairs and formed by a Metal Organic Chemical Vapor Deposition (MOCVD) or Metal Organic Vapor Phase Epitaxy (MOVPE).
Next, as shown in FIG. 2, a SiO2 or Si3N4 mask 20 is formed into a stripe on the upper surface of layer 18. Subsequently, the multi layered structure 100 is selectively etched down to the n-InP substrate 10 to produce a mesa stripe 50, as illustrated in FIG. 3. The mesa stripe 50, which has the mask 20 on top, is then introduced into a liquid phase epitaxial growth system or a MOCVD growth system, so that a p-InP current blocking layer 32 and an n-InP current blocking layer 34 are subsequently formed, as shown in FIG. 4. Current blocking layers 32 and 34 surround mesa stripe 50 and form a second crystal growth.
The first current blocking layer 32 may be doped with impurity ions, such as iron (Fe) or titanium (Ti), to form a semi-insulating (si) InP(Fe) blocking layer 32. The addition of Fe-impurity ions increases the resistivity of the first current blocking layer 32 and reduces the leakage current that typically occurs at the interface between the substrate 10 and the first current blocking layer 32. Similarly, the second current blocking layer 34 may be doped with impurity ions, such as silicon (Si), sulfur (Su) or tin (Sn), to form an n-type InP-doped blocking layer 34.
Referring now to FIG. 5, after removal of the mask 20 and the optional removal of the SAC-Q layer 18, a third crystal growth is performed on the upper surfaces of the second current blocking layer 34 and the SAC-Q layer 18. Thus, a p-InP burying layer 42 (also called a third clad layer) and a p-InGaAsP or a p-InGaAs ohmic contact layer 44 are further grown to form a buried heterostructure. The burying layer 42 may be also doped with p-type impurity ions, such as zinc (Zn), magnesium (Mg), or berilium (Be), to form a p-type InP-doped burying layer 42. Since Zn is the most commonly used p-type dopant, reference to the burying layer 42 will be made in this application as to layer InP(Zn)-doped.
Next, as illustrated in FIG. 6, an n-type electrode 62 is formed on the lower surface of semiconductor substrate 10 and a p-type electrode 64 is formed on the upper surface of the ohmic contact layer 44. Thus, a buried heterostructure laser diode is fabricated in accordance with the above described method.
A problem that occurs in the method of fabricating the above structure is the iron-zinc (Fexe2x80x94Zn) interdiffusion at the interface between the semi-insulating p-InP (Fe) first current blocking layer 32 and the p-InP(Zn) burying layer 42. The problem arises because the Fe-doped InP current blocking layer 32, which was initially covered by the mask 20, comes in contact with the Zn-doped InP burying layer 42 after the removal of the mask 20. The contact region is exemplified in FIGS. 5 and 6 as regions D, situated on lateral sides of the mesa stripe 50. The dissociation of Fe and Zn atoms at the regions D, and their consequent interdiffusion, can significantly increase the leakage current and degrade the device, leading to a poor manufacturing yield. In addition, if the active layer 14 has a multiple quantum well (MQW) structure, the Zn impurities in the Zn-doped InP burying layer 42 can enter the active layer 14 to form mixed crystals therein and practically reduce the quantum effect to zero.
In an effort to suppress the interdiffusion of dopant atoms, such as those of Zn and Fe, different techniques have been introduced in the IC fabrication. For example, one technique of the prior art, exemplified in FIG. 7, contemplates the insertion of an intrinsic or undoped InP layer 70 between the Fe-doped InP current blocking layer 32 and the Zn-doped InP burying layer 42, to prevent the contact between the InP(Fe) layer and InP(Zn) layer and to eliminate the iron-zinc interdiffusion and the consequent leakage current. This technique, however, has a major drawback in that it affects the p-n junction between the n-InP second current blocking layer 34 and the p-InP burying layer 42. Specifically, the addition of an intrinsic InP layer modifies the p-n junction that should be in the active region of devices like laser, and creates instead a p-i-n junction that alters the device characteristics altogether.
Accordingly, a method for forming a mesa stripe for buried heterostructure laser diodes, which is inexpensive to implement, and capable of decreasing the leakage current and the interdiffusion of dopant atoms, is needed. There is also a need for such a semiconductor device having good operating characteristics with reduced impurity atoms interdiffusion, reduced leakage current, and which has improved accuracy and operation reliability.
The present invention provides a method for reducing the interdiffusion between doped regions of semi-insulating buried ridge structures of forward biased devices, such as lasers and optical amplifiers.
The present invention utilizes a double dielectric mask that can be selectively etched. The mesa is undercut and an InP(Fe) layer grown. Next, the first mask is partially etched and a Si-doped InP layer is selectively grown. The second mask is subsequently etched and an InP(Zn) clad layer, along with a Zn-doped InGaAs contact layer, are grown. This way, no contact between the InP(Zn) clad layer and the InP(Fe) layer is formed, and the Znxe2x80x94Fe interdiffusion is suppressed.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiment which is provided in connection with the accompanying drawings.